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Zambales Ophiolite
Lithology Peridotite , gabbro , diabasic dike complex , diorite , pillow basalt , pelagic limestone , sandstone , mudstone Stratigraphic relations Constitutes the basement of Zambales Distribution Occupies the greater part of Zambales province; Mayantoc, Tarlac; Barlo and Sual, Pangasinan Age Eocene Previous name Zambales Ultramafic Complex (Stoll, 1962) Renamed by BMG (1981) as Zambales Ophiolite Description The Zambales Ophiolite Complex, an east-dipping complete sequence of oceanic crust and mantle material, is located at the western portion of Central Luzon spanning 160 km N-S and 40 km at its widest (E-W) portion. Three structural massifs separated by WNW trending faults are recognized: Masinloc, Cabangan and San Antonio. Petrological and geochemical studies reveal that this ophiolite suite can be divided into two major units, the Coto and Acoje blocks (Hawkins and Evans, 1983; Yumul, 1989, 1990), each made up of a complete ophiolite sequence: residual (upper mantle) - transition zone – cumulate (base of crust) - volcanic (oceanic crust) rocks. The Acoje Block occupies the northern half of the Masinloc Masif, and also the San Antonio Massif at the southern portion of the Zambales Range. The Coto Block encompasses the Cabangan Massif and the southern half of the Masinloc Massif. An Eocene age has been assigned to this ophiolite, based on overlying Eocene pelagic limestones of the Aksitero Formation (Villones, 1980; Scweller and others, 1983). This is supported by whole rock K-Ar ages of 46.6 ± 5.1 Ma and 44.3 ± 3.5 Ma for a dike at Coto mine and a sill in Sual, Pangasinan, respectively (Fuller and others, 1991). Tectonic (Karig, 1982) and paleomagnetic (Fuller and others, 1983) studies suggest that the Zambales ophiolite is an allochthonous terrane. Stratigraphic evidences (Scweller and others, 1983), in turn, point to its emergence during the Oligocene to Early Miocene. Coto Block . This consists, from bottom to top, of metamorphic harzburgite, dunite, troctolite, allivalite, olivine gabbro and a high level plutonic-volcanic suite of diorite, diabase and basalt. Massive to banded harzburgites are mostly serpentinized and consist basically of olivine, orthopyroxene, spinel and minor clinopyroxene. The harzburgites are separated from the cumulate rock suite by a transitional zone of intensely fractured black serpentinized dunite which contain chromite lenses in places. The contact between the transiton zone dunite and cumulate gabbro is characterized by interlayering-interfingering of dunite, harzburgite and gabbro The mafic cumulates are represented by anorthosite, troctolite, and olivine gabbro. They exhibit rhythmic layering and structures such as scour and fill, graded bedding and flame structures reminiscent of soft sediments. Intrusive relationships among the higher level units - basalt, diabase and gabbro - suggest that these rocks were more or less contemporaneous. Dike boundaries are usually defined by chilling on both sides. The basalt and diabase, which show evidence of low grade greenschist facies metamorphism, have aphyric to porphyritic and intersertal to intergranular textures. Clinopyroxene is dominant over plagioclase and olivine. Disseminations of magnetite, ulvospinel and pyrite are common. The diorites/tonalites are holocrystalline to poikilitic, and are composed of plagioclase, brown amphibole, quartz and minor clinopyroxene and magnetite. Epidote and chlorite are the dominant alteration minerals. Acoje Block . The bottom to top sequence of the Acoje block consists of metamorphic harzburgite with associated lherzolite and dunite, well developed ultramafic and mafic cumulates and a high level plutonic-volcanic suite of gabbro-diorite-dolerite and basalt. The massive and intensely fractured residual harzburgites are associated with pockets of lherzolite, and are much fresher compared to their counterparts of the Coto block. Like in the Coto block, the residual-cumulate transition is marked by a dunite layer. This dunite in Acoje, called ‘black’ dunite in the mine, has a very dark appearance probably due to abundant iron oxide dust inclusions. In addition, these dunites host chromite and nickel mineralization. Several gabbro dikes intrude the dunite. The ultramafic cumulates consist of rhythmically layered clinopyroxenites, dunites, wehrlites and harzburgites. Olivine, spinel and pyroxene are the main minerals, although completely altered plagioclases are very sparsely present. Talc, serpentine and iron oxide stains are very common. The mafic cumulates - gabbro, eucrite, gabbronorite and anorthosite - exhibit normal and reverse graded bedding and other sedimentary structures that include chanelling, scour and fill, slumping and flame structures. The rocks are mostly medium- to coarse-grained, consisting chiefly of plagioclase, pyroxene and olivine. Unlike in the Coto block, orthopyroxene is an important cumulus phase in the Acoje block. The diorite-diabase sill/dike complex associated with basalts are best exposed in the Barlo-Sual area. The basalt-dolerite units are usually aphanitic and greenish gray and slightly chloritized and argillized. The diorites are fine- to medium-grained and grayish-white in color. The basaltic flows and pillow basalts in Barlo are host to Cyprus-type massive sulfide deposit. The origin of the Zambales Ophiolite Complex is still open to debate. Yumul and others (1990b,c) advocate a South China Sea Basin origin for the ophiolite, whereas earlier workers (Hawkins and Evans, 1983; Fuller and others, 1983 and Schweller and others, 1983) do not favor such a hypothesis. Nicolas and Violette (1982), based on petrofabric studies, concluded that the Acoje Block is characterized by vertical plastic flow patterns indicating a mid-ocean spreading ridge environment. The Coto Block on the other hand, has subhorizontal plastic flow patterns, which are supposed to indicate its remoteness from the spreading ridge. Finding no volcanic or continental materials in the overlying hemipelagic Aksitero Formation, Schweller and others (1983) were convinced that the ophiolite was formed in an open ocean environment far from any continent or island arc. The petrology and geochemistry of the Coto and Acoje blocks are distinct from each other. While the former has geochemical characteristics transitional between normal MORB and island arc rocks, the latter is principally island arc in character. To account for such peculiarity, Hawkins (1980) and Hawkins and Evans (1983) proposed a paired volcanic arc-back-arc model, in which one component was tectonically juxtaposed against the other. Geary and others (1989), in turn, suggested that the Zambales ophiolite preserves a normal ocean crust (forearc)-island arc complex that was formed by two distinct crustal generation events. Mitchell and others (1986) offered a model in which island arc volcanics truncated and formed on top of a MORB crust. Finally, Yumul (1987) believes that the Zambales Ophiolite originated as a Paleogene subduction-related marginal basin, a precursor to the South China Sea basin, rifted from the Asia mainland. He forwarded two models, one in which the Coto Block first formed as a marginal basin followed by the installation of an island arc (Acoje Block), or, alternatively, one in which the island arc was split to form the Coto Block back-arc basin.